Learn the important aspects of RS-422/485 system design.

Transcription

1 The Elements of an S-422 and S-485 System Learn the important aspects of S-422/485 system design. - System configuration - Cabling selection - Transient protection - Software - Device seletion S-422 and S-485 Optical Isolator/epeater S-232 S-422/485 DIN ail Converter US Hubs (S-232/422/485) blackbox.com

2 The Elements of an S-422 and S-485 System Table of Contents Chapter : Overview Introduction...4 Data Transmission Signals...4 Unbalanced Line Drivers...4 alanced Line Drivers...4 alanced Line eceivers...5 TI/EI Standard S-422 Data Transmission...5 TI/EI Standard S-485 Data Transmission...7 Tristate Control of an S-485 Device Using TS Send Data Control of an S-485 Device Chaper 2: System Configuration... Network Topologies... Two-Wire or Four-Wire Systems... Termination iasing an S-485 Network... 4 Extending the Specification...5 Chapter 3: Selecting S-422 and S-485 Cabling... 6 Number of Conductors... 6 Shielding...6 Cable Characteristics... 6 Chapter 4: Transient Protection of S-422 and S-485 Systems...8 What Does a Surge Look Like?... 8 Surge Specifications...8 Common Mode vs. Differential Mode...20 round round Transient Protection Using Isolation Isolation Theory Isolation Devices... 2 Transient Protection Using Shunting Shunting Theory...22 Connecting Signal rounds Shunting Devices...22 Combining Isolation and Shunting...23 Special Consideration for Fault Conditions Choosing the ight Protection for Your System blackbox.com Page 2

3 The Elements of an S-422 and S-485 System Chapter 5: Software Introduction s-422 Systems S-485 Driver Control S-485 eceiver Control Master Slave Systems Four-Wire Master-Slave System Two-Wire Master-Slave System Multi master S-485 System Systems with Port-Powered Converters Chapter 6: Selecting S-485 Devices...27 Chapter 7: Sources of Further Information...28 ppendix : TI/EI Specification Summary ppendix : TI/EI Standard S-423 Data Transmission... 3 bout lack ox... 3 We re here to help! If you have any questions about your application, our products, or this white paper, contact lack ox Tech Support at or go to blackbox.com and click on Talk to lack ox. You ll be live with one of our technical experts in less than 20 seconds blackbox.com Page 3

4 Chapter : Overview Chapter : Overview Introduction In this white paper, we'll describe the main elements of an S-422 and S-485 system, and we'll cover all the important aspects of data system design. Since both S-422 and S-485 are data transmission systems that use balanced differential signals, we'll discuss both systems in the same white paper. Throughout this white paper, the generic terms of S-422 and S-485 will be used to represent the TI/EI-422 and TI/EI-485 standards. Data Transmission Signals Unbalanced Line Drivers Each signal that transmits in an S-232 unbalanced data transmission system appears on the interface connector as a voltage with reference to a signal ground. For example, the transmitted data (TD) from a DTE device appears on Pin 2 with respect to Pin 7 (signal ground) on a D25 connector. This voltage will be negative if the line is idle and alternate between that negative level and a positive level when data is sent with a magnitude of ±5 to ±5 volts. The S-232 receiver typically operates within the voltage range of +3 to +2 and -3 to -2 volts as shown in Figure -. D S-232 Interface Driver Loaded Output Voltage ange = + (5 to 5 volts) Space or Logic 0 ange +3-3 Mark or Logic or Idle ange Figure - S-232 Interface Circuit Transition egion +25 volts Maximum Operating ange -25 volts eceiver ange alanced Line Drivers In a balanced differential system, the voltage produced by the driver appears across a pair of signal lines that transmit only one signal. Figure -2 shows a schematic symbol for a balanced line driver and the voltages that exist. balanced line driver will produce a voltage from 2 to 6 volts across its and output terminals and will have a signal ground (C) connection. lthough proper connection to the signal ground is important, it isn t used by a balanced line receiver in determining the logic state of the data line. balanced line driver can also have an input signal called an signal. The purpose of this signal is to connect the driver to its output terminals, and. If the signal is OFF, the driver is considered to be disconnected from the transmission line. n S-485 driver must have the control signal. n S-422 driver may have this signal, but it is not always required. The disconnected or disabled condition of the line driver usually is referred to as the tristate condition of the driver. (Optional for S-422) (equired for S-485) C Figure -2 alanced Differential Output Line Driver +6 volts Permissible ange +2 volts Voltage V -2 volts Permissible ange -6 volts. The term tristate comes from the fact that there is a third output state of an S-485 driver in addition to the output states of and blackbox.com Page 4

5 Chapter : Overview alanced Line eceivers balanced differential line receiver senses the voltage state of the transmission line across two signal input lines, and. It will also have a signal ground, C, that is necessary in making the proper interface connection. Figure -3 is a schematic symbol for a balanced differential line receiver. Figure -3 also shows the voltages that are important to the balanced line receiver. If the differential input voltage (Vab) is greater than +200 mv, the receiver will have a specific logic state on its output terminal. If the input voltage is reversed to less than -200 mv, the receiver will create the opposite logic state on its output terminal. The input voltages that a balanced line receiver must sense are shown in Figure -3. The 200 mv to 6 V range is required to allow for attenuation on the transmission line. +6 volts 2 VI 2 VI C 2Vcm +200mv -200mv +Vi -Vi Maximum Operating ange Voltage V Vcm= Input Common Mode Voltage Permissible ange for Vcmi -7v > Vcm < +7v Figure -3: alanced Differential Input Line eceiver Transition egion Typical Hysteresis = 50 mv -6 volts TI/EI Standard S-422 Data Transmission The TI/EI standard S-422- entitled Electrical Characteristics of alanced Voltage Digital Interface Circuits defines the characteristics of S-422 interface circuits. Figure -4 is a typical S-422 four-wire interface. Notice that five conductors are used. Each generator or driver can drive up to 0 receivers. The two signaling states of the line are defined as follows: a. When the terminal of the driver is negative with respect to the terminal, the line is in a binary (MK or OFF) state. b. When the terminal of the driver is positive with respect to the terminal, the line is in a binary 0 (SPCE or ON) state. Figure -5 shows the condition of the voltage of the balanced line for an S-232 to S-422 converter when the line is in the idle condition or OFF state. It also shows the relationship of the and terminals of an S-422 system and the - and + terminal markings used on many types of equipment. The terminal is equivalent to the - designation, and the terminal is equivalent to the + designation. The same relationship shown in Figure -5 also applies for S-485 systems. S-422 can withstand a common mode voltage (Vcm) of ±7 volts. Common mode voltage is defined as the mean voltage of and terminals with respect to signal ground blackbox.com Page 5

6 Chapter : Overview 00 Ω 00 Ω 00 Ω 00 Ω Optional rounding Configuration Optional rounding Configuration enerator eceiver Circuit ground or circuit common Protective ground or frame ground reen wire ground or power system ground Figure -4: Typical S-422 Four-Wire Network TD () ED TD 2 S-232 S-422 TD () LK D 7 SI. ND ED - V - V LK DVM DVM Note: Under idle conditions, it is possible to determine which terminal is and which is a. Figure -5: elationship etween TI/EI Standard and Terminals on an S-422 or S-485 Device and + and - Identification Convention blackbox.com Page 6

7 Chapter : Overview TI/EI Standard S-485 Data Transmission The S-485 standard permits a balanced transmission line to be shared in a party line or multidrop mode. s many as 32 driver/ receiver pairs can share a multidrop network. Many characteristics of the drivers and receivers are the same as S-422. The common mode voltage (Vcm) range that the driver and receiver can tolerate is expanded to +2 to -7 volts. ecause the driver can be disconnected or tristated from the line, it must withstand this range while in the tristate condition. Some S-422 drivers, even with tristate capability, will not withstand the full common mode voltage range of +2 to -7 volts. Figure -6 shows a typical two-wire multidrop network. Note that the transmission line is terminated on both ends of the line but not at drop points in the middle of the line. Termination should only be used with high data rates and long wiring runs. detailed discussion of termination can be found in Chapter 2. The signal ground line is also recommended in an S-485 system to keep the common mode voltage that the receiver must accept within the -7 to +2 range. Further discussion of grounding can be found in Chapter ft. Termination esistors at oth Ends Only X t t X 00 Ω 00 Ω 00 Ω 00 Ω X X 00 Ω 00 Ω 00 Ω 00 Ω Figure -6: Typical S-485 Two-Wire Multidrop Network enerator eceiver Circuit ground or circuit common Protective ground or frame ground reen wire ground or power system ground blackbox.com Page 7

8 Chapter : Overview Master 4000 ft. Slave X t t t t X 00 Ω 00 Ω 00 Ω 00 Ω Slave X 00 Ω 00 Ω Slave X 00 Ω 00 Ω enerator eceiver Circuit ground or circuit common Protective ground or frame ground reen wire ground or power system ground Figure -7: Typical S-485 Four-Wire Multidrop Network n S-485 network can also be connected in a four-wire mode as shown in Figure -7. Note that four data wires and an additional signal ground wire are used. In a four-wire network, it is necessary that one node be a master node and all others be slaves. The network is connected so that the master node communicates to all slave nodes. ll slave nodes communicate only with the master node. This offers advantages for equipment with mixed protocol communications. ecause slave nodes never listen to another slave response to the master, a slave node cannot reply incorrectly to another slave node blackbox.com Page 8

9 Chapter : Overview VSD VTS VSD S-232 to TTL Level Converter Integrated Circuit SD 2 D 3 D VSD (TTL) X TTL to S-485 Transceiver Integrated CKT S-485 Driver D V V VSD (TTL) V V VTS TS 4 D 7 D 0 S-485 Driver Disabled Note Note: ) Voltage here is determined by other devices on the line. 2) ll peak values of voltages are approximate. Figure -8: Timing for S-232 to S-485 Converter with TS Control of S-485 Driver and eceiver Tristate Control of an S-485 Device Using TS s discussed previously, an S-485 system must have a driver that can be disconnected from the transmission line when a particular node is not transmitting. In an S-232 to S-485 converter or an S-485 serial card, this may be accomplished using the TS control signal from an asynchronous serial port to enable the S-485 driver. To do this, connect the TS line to the S-485 driver and set the TS line to a high (logic ) state to enable the S-485 driver. Setting the TS line low (logic 0) puts the driver into the tristate condition. This, in effect, disconnects the driver from the bus enabling other nodes to transmit over the same wire pair. Figure -8 shows a timing diagram for a typical S-232 to S-485 converter. The waveforms show what happens if the VTS waveform is narrower than the data VSD. This is not the normal situation, but is shown here to illustrate the loss of a portion of the data waveform. When TS control is used, it is important to be certain that TS is set high before data is sent. lso, the TS line must then be set low after the last data bit is sent. This timing is done by the software used to control the serial port and not by the converter. When an S-485 network is connected in a two-wire multidrop party line mode, the receiver at each node will be connected to the line (see Figure -6). The receiver can often be configured to receive an echo of its own data transmission. This is desirable in some systems and troublesome in others. e sure to check the data sheet for your converter to determine how the receiver enable function is connected blackbox.com Page 9

10 Chapter : Overview Send Data Control of an S-485 Device Many S-232 to S-485 converters and S-485 serial cards include special circuitry, which is triggered from the data signal to enable the S-485 driver. Figure -9 is a timing diagram of the important signals used to control a converter of this type. It is important to note that the transmit data line is disabled at a fixed interval after the last bit, typically one character length. If this interval is too short, you can miss parts of each character being sent. If this time is too long, your system may try to turn the data line around from transmit to receive before the node (with the Send Data converter) is ready to receive data. If the latter is the case, you will miss portions (or complete characters) at the beginning of a response. VSD VSD (TTL) VSD S-232 to TTL Level Converter Integrated Circuit SD 2 D 3 D VSD (TTL) X TTL to S-485 Transceiver Integrated CKT. S-485 Driver D V V V 5 0 (Note 2) T V D V V D 7 etriggerable Timing Circuit 0 S-485 Driver Disabled Note Note: ) Voltage here is determined by other devices on the line. 2) This timing interval determined by components in the timing circuit. The start of this interval is determined by the leading edge of each data bit. 3) ll peak values of voltages are approximate. Figure -9: Timing for S-232 to S-485 Converter with Send Data (SD) Control of the S-485 Driver and eceiver blackbox.com Page 0

11 Chapter 2: System Configuration Chapter 2: System Configuration Network Topologies Network configuration isn t defined in the S-422 or S-485 specification. In most cases, the network designer can use a configuration that best fits the physical requirements of the system. Two-Wire and Four-Wire Systems S-422 systems require a dedicated pair of wires for each signal: a transmit pair, a receive pair, and an additional pair for each handshake/control signal used (if required). The tristate capabilities of S-485 enable a single pair of wires to share transmit and receive signals for half-duplex communications. This two-wire configuration reduces cabling costs, but note that an additional ground conductor should be used. S-485 devices may be internally or externally configured for two-wire systems. Internally configured S-485 devices simply provide and connections, sometimes labeled - and +. Devices configured for four-wire communications provide and connections for both the transmit and the receive pairs. The user can connect the transmit lines to the receive lines to create a two-wire configuration. The latter provides the system designer with the most configuration flexibility. Note that the signal ground line should also be connected in the system. This is necessary to keep the common mode voltage at the receiver within a safe range. The interface circuit may operate without the signal ground connection but may lose reliability and noise immunity. Figures 2- and 2-2 illustrate connections of two- and four-wire systems blackbox.com Page

12 Chapter 2: System Configuration Master 4000 ft. Slave X t t t X 00 Ω 00 Ω 00 Ω 00 Ω Slave X 00 Ω 00 Ω Slave X 00 Ω 00 Ω enerator eceiver Circuit ground or circuit common Protective ground or frame ground reen wire ground or power system ground Figure 2-: Typical S-485 Four-Wire Multigroup Configuration 4000 ft. Termination esistors at oth Ends t t X X 00 Ω 00 Ω 00 Ω 00 Ω X 00 Ω 00 Ω X Figure 2-2: Typical S-485 Two-Wire Multidrop Network 00 Ω 00 Ω enerator eceiver Circuit ground or circuit common Protective ground or frame ground reen wire ground or power system ground blackbox.com Page 2

13 Chapter 2: System Configuration Termination Termination is used to match the impedance of a node to the impedance of the transmission line being used. When impedance is mismatched, the transmitted signal is not completely absorbed by the load, and a portion is reflected back into the transmission line. If the source, transmission line, and load impedance are equal, these reflections are eliminated. There are disadvantages of termination as well. Termination increases load on the drivers, increases installation complexity, changes biasing requirements, and makes system modification more difficult. The decision on whether to use termination should be based on the cable length and data rate. good rule of thumb is if the propagation delay on the data line is much less than one bit width, termination is not needed. This rule makes the assumption that reflections will damp out in several trips up and down the data line. Since the receiving UT will sample the data in the middle of the bit, it is important that the signal level be solid at that point. For example, in a system with 2000 feet of data line, the propagation delay can be calculated by multiplying the cable length by the propagation velocity of the cable. This value, typically 66 to 75% of the speed of light (c), is specified by the cable manufacturer. For our example, a round trip covers 4000 feet of cable. Using a propagation velocity of 0.66 c, one round trip is completed in approximately 6.6 μs. If we assume the reflections will damp out in three round trips up and down the cable length, the signal will stabilize at 8.5 μs after the leading edge of a bit. t 9600-baud, one bit is 04-μs wide. Since the reflections are damped out much before the center of the bit, termination is not required. There are several methods of terminating data lines. The method we recommend is parallel termination. resistor is added in parallel with the receiver s and lines to match the data line characteristic impedance specified by the cable manufacturer (20 ohm is a common value). This value describes the intrinsic impedance of the transmission line and is not a function of the line length. terminating resistor of less than 90-ohm should not be used. Termination resistors should be placed only at the extreme ends of the data line, and no more than two terminations should be placed in any system that does not use repeaters. This type of termination adds heavy DC loading to a system and may overload port-powered S-232 to S-485 converters. nother type of termination, C-coupled termination, adds a small capacitor in series with the termination resistor to eliminate the DC loading effect. lthough this method eliminates DC loading, capacitor selection is highly dependent on the system properties. System designers interested in C termination are encouraged to read National Semiconductor s pplication Note for further information. Figure 2-3 illustrates both parallel and C termination on an S-485 two-wire node. In four-wire systems, the termination is placed across the receiver of the node. X T 20 ohms X T 20 ohms C 00 nf Parallel Termination C-Coupled Termination Figure 2-3: Parallel and C Termination 2. efer to Chapter 7 for information on National Semiconductor s pplication Notes blackbox.com Page 3

14 Chapter 2: System Configuration iasing an S-485 Network When an S-485 network is in an idle state, all nodes are in listen (receive) mode. Under this condition, there are no active drivers on the network. ll drivers are tristated. Without anything driving the network, the state of the line is unknown. If the voltage level at the receiver s and inputs is less than ±200 mv, the logic level at the output of the receivers will be the value of the last bit received. To maintain the proper idle voltage state, bias resistors must be applied to force the data lines to the idle condition. ias resistors are nothing more than a pullup resistor on the data line (typically to 5 volts) and a pulldown (to ground) on the data line. Figure 2-4 illustrates the placement of bias resistors on a transceiver in a two-wire configuration. Note that in an S-485 four-wire configuration, the bias resistors should be placed on the receiver lines. The value of the bias resistors is dependent on termination and number of nodes in the system. The goal is to generate enough DC bias current in the network to maintain a minimum of 200 mv between the and data line. Consider the following two examples of bias resistor calculation: ias esistor X 2 k ias esistor +5 V Figure 2-4: Transceiver with ias esistors Example. 0-Node, S-485 Network with Two 20-Ohm Termination esistors Each S-485 node has a load impedance of 2 Kohms. 0 nodes in parallel give a load of 200 ohms. dditionally, the two 20-ohm termination resistors result in another 60-ohm load, for a total load of 57 ohms. Clearly the termination resistors are responsible for a majority of the loading. To maintain at least 200 mv between the and line, we need a bias current of 3.5 m to flow through the load. To create this bias from a 5-V supply, a total series resistance of 428 ohms or less is required. Subtract the 57 ohms that is already a part of the load, and we are left with 37 ohms. Placing half of this value as a pullup to 5 V and half as a pulldown to ground gives a maximum bias resistor value of 685 ohms for each of the two biasing resistors blackbox.com Page 4

15 Chapter 2: System Configuration Example Node, S-485 Network without Termination Each S-485 node has a load impedance of 2 Kohms. 32 nodes in parallel give a total load of 375 ohms. To maintain at least 200 mv across 375 ohms, we need a current of 0.53 m. To generate this current from a 5-V supply requires a total resistance of 9375 ohms maximum. Since 375 ohms of this total is in the receiver load, our bias resistors must add to 9 Kohms or less. Notice that very little bias current is required in systems without termination. ias resistors can be placed anywhere in the network or can be split among multiple nodes. The parallel combination of all bias resistors in a system must be equal to or less than the calculated biasing requirements. lack ox uses bias resistors in its S-485 products, most with a value of 4.7 Kohms. This value is adequate for most systems without termination. You should always calculate the biasing requirements of the network. Symptoms of underbiasing range from decreased noise immunity to complete data failure. Overbiasing has less effect on a system; the primary result is increased load on the drivers. Systems using portpowered S-232 to S-485 converters can be sensitive to overbiasing. Extending the Specification Some systems require longer distances or higher numbers of nodes than supported by S-422 or S-485. epeaters are commonly used to overcome these barriers. n S-485 repeater, such as the lack ox IC650-US, can be placed in a system to divide the load into multiple segments. Each refreshed signal is capable of driving another 4000 feet of cable and an additional 3 S-485 loads. nother method of increasing the number of S-485 nodes is to use low-load-type S-485 receivers. These receivers use a higher input impedance to reduce the load on the S-485 drivers to increase the total number of nodes. There are currently half- and quarter-load integrated circuit receivers available, extending the total allowable number of nodes to 64 and blackbox.com Page 5

16 Chapter 3: Selecting S-422 and S-485 Cabling Chapter 3: Selecting S-422 and S-485 Cabling Cable selection for S-422 and S-485 systems is often neglected. ttention to a few details in the selection process can prevent the costly prospect of repulling thousands of feet of cable. Number of Conductors The signal ground conductor is often overlooked when ordering cable. n extra twisted pair must be specified to have enough conductors to run a signal ground. two-wire system then requires two twisted pairs and a four-wire system requires three twisted pairs. Shielding It is often hard to quantify if shielded cable is required in an application or not. Since the added cost of shielded cable is usually minimal, it is worth installing the first time. Cable Characteristics When choosing a transmission line for S-422 or S-485, it is necessary to examine the required distance of the cable and the data rate of the system. The ppendix to TI/EI S-422- standard presents an empirical curve that relates cable length to data rate for 24 W, twisted-pair telephone cable that has a shunt capacitance of 6 pf/ft. and is terminated in 00 ohms (see Figure 3-). This curve is based on signal quality requirements of: a. Signal rise and fall time equal to, or less than, one-half unit interval at the applicable modulation rate. b. The maximum voltage loss between driver and load of 6 d. 0K 4K Cable Length Feet K K 00K M 0M Data ate its/s Figure 3.: Data Signaling ate Versus Cable Length for alanced Interface Using 24 W, Twisted-Pair Cable blackbox.com Page 6

17 Chapter 3: Selecting S-422 and S-485 Cabling 4K ttenuation d/00 ft PVC Polyethylene Foamed Cellular Polyethylene Frequency MHz Figure 3-2: ttenuation versus Frequency for Several Data Cables Losses in a transmission line are a combination of C losses (skin effect), DC conductor loss, leakage, and C losses in the dielectric. In high-quality cable, the conductor losses and the dielectric losses are on the same order of magnitude. Figure 3-2 is included to point out the significant difference in performance of different cables. This chart shows ttenuation versus Frequency for three different cables. Note that the polyethylene cables offer much lower attenuation than PVC cables blackbox.com Page 7

18 Chapter 4: Transient Protection of S-422 and S-485 Systems Chapter 4: Transient Protection of S-422 and S-485 Systems The first step towards protecting an S-422 or S-485 system from transients is understanding the nature of the energy we are guarding against. Transient energy may come from several sources, most typically environmental conditions or induced by switching heavy inductive loads. What Does a Surge Look Like? Surge Specifications While transients may not always conform to industry specifications, both the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE) have developed transient models for use in evaluating electrical and electronic equipment for immunity to surges. These models can offer some insight into the types of energy that must be controlled to prevent system damage. oth the IEC : 995 Surge Immunity Test and the IEEE C IEEE ecommended Practice on Surge Voltages in Low-Voltage C Power Circuits define a.2/50-μs 8/20-μs combination wave surge, which has a.2-μs voltage rise time with a 50-μs decay across an open circuit. The specified current waveform has an 8-μs rise time with a 20-μs decay into a short circuit. Open circuit voltages levels from to 6 kv are commonly used in both the positive and negative polarities, although under some circumstances, voltages as high as 20 kv may be applied. Figures 4- and 4-2 illustrate the combination wave characteristics. In addition, IEEE C62.4 also specifies a 00-kHz ring wave test. The ring wave has a 0.5-μs rise time and a decaying oscillation at 00 khz with source impedance of 2 as shown in Figure 4-3. Typical amplitudes for the 00-kHz ring wave also range from.2/50-µs Second Voltage Wave V(t) / Vp Time, µs Figure 4-: Combination Wave Voltage Waveform blackbox.com Page 8

19 Chapter 4: Transient Protection of S-422 and S-485 Systems 8/20-µs Current Wave V(t) / Vp Time, µs Figure 4-2: Combination Wave Current Waveform 00-kHz ing Wave V(t) / Vp Time, µs Figure 4-3: 00-kHz ing Wave blackbox.com Page 9

20 Chapter 4: Transient Protection of S-422 and S-485 Systems 6 kv. Common Mode vs. Differential Mode Identifying the type of surges that may threaten a system is an important part of selecting the appropriate levels and methods of transient protection. Since each of the conductors in a data cable travels through the same physical space, it is reasonable to expect transients caused by environmental or current switching to be common mode, that is, present on all data and ground conductors within the data cable. In some installations, there may be another source of unwanted energy to consider. If there are high-voltage cables running anywhere near the data cables, the potential for a fault condition exists as a result of insulation failures or inadvertent contact by an installer. This type of surge could contact any number of conductors in the data cable, presenting a differential surge to the data equipment. lthough the voltages and currents associated with this type surge are much lower than the types of surges modeled by NSI or IEC, they have a particularly destructive quality of their own. Instead of dissipating within several milliseconds, they can exist in a steady state condition on the data network. round round ealizing that transient energy can be high frequency in nature leads to some disturbing observations. t frequencies of this magnitude, it is difficult to make a low impedance electrical connection between two points because of the inductance of the path between them. Whether that path is several feet of cable or thousands of feet of earth between grounding systems, during a transient event there can be hundreds or thousands of volts potential between different grounds. We can no longer assume that two points connected by a wire will be at the same voltage potential. To the system designer, this means that although S-422/485 uses 5-V differential signaling, a remote node may see the 5-V signal superimposed on a transient of hundreds or thousands of volts with respect to that node s local ground. It is more intuitive to refer to what is commonly called signal ground as a signal reference. How do we connect system nodes knowing that these large potential differences between grounds may exist? The first step is to ensure that each device in the system is referenced to only one ground, eliminating the path through the device for surge currents searching for a return. There are two approaches to creating this idyllic ground state: The first approach is to isolate the data ground from the host device ground. This is typically done with transformers or optical isolators as shown is Figure 4-4. The second approach is to tie each of the grounds on a device together (typically power ground and data ground) with a low Device Vcc Port Isolated Power Data Lines Out Optical Isolation Figure 4-4 : Isolated S-485 Device blackbox.com Page 20

21 Chapter 4: Transient Protection of S-422 and S-485 Systems Device Vcc Port Data Lines Out round Line Local Chassis round Connection Figure 4-5: S-485 Device with Signal round Connected to Chassis round impedance connection as shown in Figure 4-5. These two techniques lead us to the two basic methods of transient protection. Transient Protection Using Isolation Isolation Theory The most universal approach to protecting against transients is to galvanically isolate the data port from the host device circuitry. This method separates the signal reference from any fixed ground. Optical isolators, transformers, and fiber optics are all methods commonly used in many types of data networks to isolate I/O circuitry from its host device. In S-422 and S-485 applications, optical isolators are most common. n optical isolator is an integrated circuit that converts the electrical signal to light and back, eliminating electrical continuity. With an isolated port, the entire isolated circuitry floats to the level of the transient without disrupting data communications. s long as the floating level of the circuitry does not exceed the breakdown rating of the isolators, typically volts, the port will not be damaged. This type of protection does not attempt to absorb or shunt excess energy so it is not sensitive to the length of the transient. Even continuous potential differences will not harm isolated devices. It is important to note that while isolators work on common mode transients, they cannot protect against large voltage differences between conductors of a data cable, such as those caused by short circuits between data and power circuits. Isolation Devices Optical isolation can be implemented in a number of ways. If a conversion from S-232 to S-422 or S-485 is being made, optically isolated converters are available. Optically isolated IS bus serial cards can replace existing ports in PC systems. For systems with existing S-422 or S-485 ports, an optically isolated repeater can be installed. Examples of each of these type of devices can be found at blackbox.com blackbox.com Page 2

22 Chapter 4: Transient Protection of S-422 and S-485 Systems Transient Protection using Shunting Shunting Theory Creating one common ground at the host device provides a safe place to divert surge energy as well as a voltage reference for attaching surge suppression devices. Shunting harmful currents to ground before they reach the data port is the job of components such as transient voltage suppression diode (TVS), often referred to as a TranZorb or transorb, MOV, or gas discharge tubes. These devices all work by clamping at a set voltage. Once the clamp voltage has been exceeded, the devices provide a low impedance connection between terminals. ecause a shunting device diverts a large amount of energy, it cannot tolerate very long duration or continuous transients. Shunting devices are most often installed from each data line to the local earth ground. They should be selected to begin conducting current at a voltage as close as possible above the system's normal communications levels. For S-422 and S-485 systems, the range is typically 6 8 volts. These devices usually add some capacitive load to the data lines. That should be considered when designing a system and can be done by derating the total line length to compensate for the added load. Several hundred feet is normally adequate. To use shunting devices correctly, they should be installed as close as possible to the port to be protected. The user must also provide an extremely low impedance connection to the local earth ground of the unit being protected. This ground connection is crucial to the proper operation of the shunting device. The ground connection should be made with heavy gauge wire and kept as short as possible. If the cable is longer than one meter, copper strap or braided cable intended for grounding purposes must be used for the protection device to be effective. In addition to the high frequency nature of transients, there can be an enormous amount of current present. Several thousand amps typically result from applications of the combination wave test in the NSI and IEC specification. Connecting Signal rounds Since a local ground connection is required at each node using shunt protection, the consequences of connecting remote grounds together must be considered. During transient events, the potential for high voltage exists between the remote grounds. Only the impedance in the wire connecting the grounds limits the current from this voltage. The S-422 and S-485 specification both recommend using 00-ohm resistors in series with the signal ground path to limit ground currents. Figure 4-6 illustrates the X X 00 Ω 00 Ω Earth round Earth round Figure 4-6: Signal round Connection between Two Nodes with 00-Ohm esistor ground connection recommended in the specification. Shunting Devices There are two types of shunting devices to choose from: The least expensive type is single stage, which usually consists of a single TVS device on each line. Three-stage devices are also available. The first stage of a three-stage device is a gas discharge tube, which can handle extremely high currents, but has a high threshold voltage and is too slow to protect solid-state circuits. The second stage is a small series impedance, which limits current and creates a voltage drop between the first and third stage. The final stage is a TVS device that is fast enough to protect solid-state devices and brings the clamping voltage down to a safe blackbox.com Page 22

23 Chapter 4: Transient Protection of S-422 and S-485 Systems Device Vcc Isolated Power Port Shunting Device Data Lines Out round Line Earth ound Figure 4-7: Isolated Node with Shunt Protection to Earth round Device Vcc Port Isolated Power Shunting Device Data Lines Signal round Figure 4-8: Isolated Port with Underground Shunt Protection level for data circuits. Combining Isolation and Shunting Installing a combination of both types of protection can offer the highest reliability in a system. Figures 4-7 and 4-8 illustrate two means of implementing this level of protection. The method shown in Figure 4-7 is recommended. In this case, isolation protects the circuit from any voltage drops in the earth ground connection. The shunt device prevents a surge from exceeding the breakdown voltage of the isolators as well as handling any differential surges on the cable. Figure 4-8 illustrates a method recommended for cases where there is no way to make an earth ground connection. Here, the shunt device s function is to protect the port from differential surges. differential surge will be balanced between conductors by the shunting device, converted to common mode. The isolation provides protection from the blackbox.com Page 23

24 Chapter 4: Transient Protection of S-422 and S-485 Systems Device Vcc Data Lines 25 m Fuse Signal round Figure 4-9 Fused port protection Earth round common mode transient remaining. Special Consideration for Fault Conditions Data systems that can be affected by short circuits to power conductors require an extra measure of protection. In these cases, it s recommended to add a fuse-type device in addition to shunting suppression, as shown in Figure 4-9. When a short circuit occurs, the shunt suppression will begin conducting, but shunting itself cannot withstand the steady state of currents this type of surge produces. small fuse value should be chosen so that the fuse will open before the shunt device is damaged. typical fuse value is 25 m. Choosing the ight Protection for Your System While it is hard to predict what type and level of isolation is correct for a system, an educated guess should be made based on the electrical environment, physical conditions, and the cost of failures in downtime and repairs. Systems connected between two power sources, such as building to building, office to factory floor, or any system covering long distances should have some level of transient protection. Table 4- is a comparison of transient protection techniques. Table 4- Comparison of Protection Techniques Optical Isolation Shunting equires no ground reference Must have low impedance ground path dds no loading to data lines Higher complexity Effective on common mode transients Not dependent on installation quality equires an external power source Not affected by long term or continuous transients Presents additional capacitive loading to data lines Lower complexity, uses passive components Effective on both common and differential mode transients Can be improperly installed by user No power required Subject to damage by long duration transients blackbox.com Page 24

25 Chapter 5: Software Chapter 5: Software Introduction S-422 and S-485 are hardware specifications. Software protocol is not discussed in either specification. It is up to the system designer to define a suitable protocol. In this chapter, we will not attempt to define a protocol standard, but will explain some of the issues that you should consider when writing or purchasing software. S-422 Systems S-422 system software differs little from the familiar point-to-point S-232 communication systems. S-422 is often used to simply extend the distance between nodes over the capabilities of S-232. S-422 can also be used as the master node in a fourwire master-slave network described later in this chapter. When selecting or writing software for S-422 systems, the designer should be aware of the signals being used by the hardware in the system. Many S-422 systems do not implement the hardware handshake lines often found in S-232 systems because of the cost of running additional conductors over long distances. S-485 Driver Control The principle difference between S-422 and S-485 is that the S-485 driver can be put into a high impedance, tristate mode, which allows other drivers to transmit over the same pair of wires. There are two methods of tristating an S-485 driver. The first method is to use a control line, often the TS handshake line, to enable and disable the driver. This requires that the host software raise the TS line before beginning a transmission to enable the driver, then lower the TS line after the completion of the transmission. ecause only a single S-485 driver can be enabled on a network at a time, it is important that the driver is disabled as quickly as possible after transmission. This prevents the two drivers from trying to control the lines simultaneously, a condition called line contention. In some operating systems, it can be difficult to lower TS in a timely manner and this method of driver control should be avoided altogether. The second method of S-485 driver control is utomatic Send Data control. This involves special circuitry that senses when data is being transmitted. It automatically enables and disables the driver within one character length of the end of transmission. This is the preferred method of driver control because it reduces software overhead and the number of potential pitfalls for the programmer. S-485 eceiver Control The S-485 receiver also has an enable signal. In S-485 systems using a two-wire configuration to connect the driver to receiver in a loopback, this feature is often used to disable the receiver during transmission to prevent the echo of local data. nother approach is to leave the S-485 receiver enabled and monitor the loopback data for errors, which would indicate that line contention has occurred. lthough a good loopback signal does not guarantee data integrity, it does offer a degree of error detection. Master-Slave Systems master-slave system has one node that issues commands to each of the slave nodes and processes responses. Slave nodes will not typically transmit data without a request from the master node and they do not communicate with each other. Each slave must have a unique address so that it can be addressed independently of other nodes. Master-slave systems can be configured as two-wire or four-wire. Four-wire systems often use an S-422 master (the driver is always enabled) and S-485 slaves to reduce system complexity blackbox.com Page 25

26 Chapter 5: Software Four-Wire Master-Slave System This configuration reduces software complexity at the host because the driver and receiver are always enabled. ut this happens at the expense of installing two extra conductors in the system. The Master node simply prefixes commands with the appropriate address of the slave. There are no data echo or turnaround delays to consider. ecause each of the slave transmitters share the same pair of wires, care must be taken that the master never requests data from multiple nodes simultaneously or data collisions will result. Two-Wire Master-Slave System Two-wire configurations add a small amount of complexity to the system. The S-485 driver must be tristated when not in use. This enables other nodes to use the shared pair of wires. The time delay between the end of a transmission and the tristate condition is very important in a two-wire master-slave system. If a slave attempts to reply before the master has tristated the line, a collision will occur and data will be lost. The system designer must know the response time or turnaround delay of each of the slave nodes and ensure that the master will tristate its driver within that amount of time. Multimaster S-485 System Each node in a multimaster S-485 system can initiate its own transmission creating the potential for data collisions. This requires the designer to use a more sophisticated method of error detection, including line contention detection, acknowledgement of transmissions, and resending corrupted data. Systems with Port-Powered Converters S-232 to S-422 or S-485 converters that derive power from the S-232 port are becoming more common. good programming practice is to set unused handshake outputs to a high voltage state. This ensures the best possible operating conditions for all converters used blackbox.com Page 26

27 Chapter 6: Selecting S-485 Devices Chapter 6: Selecting S-485 Devices When purchasing devices for an S-485 system, the following communications characteristics should be considered in the system design stage to avoid later pitfalls.. Is the device configured for two-wire or four-wire systems? 2. Is a signal ground connection available? 3. Is the device isolated? Does it contain surge suppression? 4. What value bias resistors (if any) are used in the device? re they accessible for modification? 5. Is the device terminated? Is it accessible for modification? 6. What is the device s response time (turnaround delay)? 7. What is the programmable address range of the device? 8. What baud rate, or range of baud rates, is supported? If possible, it is useful to have a schematic of the serial port of each device in a system. The schematic can provide additional information that may be useful in troubleshooting problems in the data system blackbox.com Page 27

28 Chapter 7: Sources of Further Information Chapter 7: Sources of Further Information TI/EI standards and publications can be purchased from: LOL ENINEEIN DOCUMENTS 5 Inverness Way East MS 0 Englewood, CO 802 Phone: (800) ; (303) Fax: (303) Web: elated data interface standards are: TI/EI-232-E TI/EI-422- TI/EI-423- TI/EI-485 TI/EI-449 TI/EI-530 EI/TI-562 Interface between data terminal equipment and data circuit-terminating equipment using serial binary data interchange (NSI/IE-232-D) Electrical characteristics of balanced voltage digital interface circuits Electrical characteristics of unbalanced voltage digital interface circuits Standard for electrical characteristics of generators and receivers for use in balanced digital multipoint systems eneral-purpose, 37-position and 9-position interface for data terminal equipment and data circuit-terminating equipment High-speed, 25-position interface for data terminal equipment and data circuit-terminating equipment Electrical characteristics for an unbalanced digital interface Manufacturers of integrated circuit data transceivers often offer practical application information for S-422 and S-485 systems. National Semiconductor has many application notes that are available on-line at Enter 422 or 485 in the search field blackbox.com Page 28

29 ppendix : TI/EI Specification Summary ppendix : TI/EI Specification Summary Parameter Conditions Min. Max. Units Driver Output Voltage 0 V Open Circuit -0 V Driver Output Voltage 2 V Loaded T =00 Ω -2 V Driver Output esistance to 00 Ω Driver Output Short- Circuit Current Per output to common ±50 m Driver Output % of bit ise Time T =00 Ω 0 width Driver Common Mode Voltage T =00 Ω ±3 V eceiver Sensitivity Vcm ± 7 ±200 mv eceiver Common-Mode Voltage ange TI/EI S-422 Specifications Summary V eceiver Input esistance 4000 Ω Differential eceiver Operational: ±0 V Voltage Withstand: ±2 V Parameter Conditions Min. Max. Units Driver Output Voltage.5 6 V Open Circuit V Driver Output.5 5 V Voltage Loaded LOD =54 Ω V Driver Output Short- Per output to Circuit Current +2V or -7V ±250 m Driver Output LOD=54 Ω 30 % of bit ise Time C LOD=50 pf width Driver Commom Mode Voltage LOD =54 Ω - 3 V eceiver Sensitivity -7 Vcm ±2 ±200 mv eceiver Common- Mode Voltage ange TI/EI S-485 Specifications Summary -7 ±2 V eceiver Input esistance 2K Ω blackbox.com Page 29

30 ppendix : TI/EI Specification Summary TI/EI S-232 Specification Summary Parameter Conditions Min. Max. Units Driver Output Voltage Open Circuit 25 V Driver Output 3 K Ω L 7 V Voltage Loaded K Ω 5 5 V Driver Output esistance Power Off -2V Vo 2V 300 Ω Driver Output Short-Circuit Current 500 m Driver Output Slew ate 30 V/ s Maximum Load Capacitance 2500 pf eceiver Input esistance 3V VIN 25V Ω eceiver Input esistance Output = Mark -3 V Output = Space 3 V TI/EI S-423 Specification Summary Parameter Conditions Min. Max. Units Driver Output Voltage 4 6 V Open Circuit -4-6 V Driver Output Voltage Loaded L = 450 Ω V Driver Output esistance -2V Vo 2V 50 Ω Driver Output Short-Circuit Current ±50 m Driver Output ise aud ate K aud 300 µs % Unit and Fall Time aud ate K aud 30 Interval eceiver Sensitivity Vcm ±7V ±200 mv eceiver Input esistance 4000 Ω blackbox.com Page 30

31 ppendix : TI/EI Standard S-423 Data Transmission ppendix : TI/EI Standard S-423 Data Transmission S-423 (TI/EI-423) is another standard used in point-to-point communications. S-423 data transmission uses an unbalanced line driver that connects to an S-422 type balanced line receiver as shown in Figure -. The S-423 line driver is unique to this system. It produces voltage similar to S-232 but has a slew rate control input that is used to limit rise times and crosstalk on the data lines. Typical adjustment on the slew rate control is from to 00 μs. This is done by the selection of one resistor on the wave shape control input. S-423 Diver is Unique to S-423 s Slew ate Control D S-423 Interface Vg Driver Loaded Output Voltage ange = 3.6 to 6 volts S-423 Users Same eceiver as S mv -200mv +6 volts Maximun Operating ange Transition egion -6 volts eceiver Voltage V Figure -: S-423 Interface Circuit bout lack ox lack ox Network Services is a leading datacom products and converters provider, serving 75,000 clients in 4 countries with 93 offices around the world. You ll find more than 8,000 products, including S-22 and S-485 products, in the lack ox Catalog and at blackbox.com. lack ox also offers cabling, switches, routers, cabinets and racks, KVM, digital signage, and networking products all supported by free, live 24/7 technical support. Copyright ll rights reserved. lack ox and the Double Diamond logo are registered trademarks of Technologies, Inc. ny third-party trademarks appearing in this white paper are acknowledged to be the property of their respective owners blackbox.com Page 3